The present invention relates to a power steering system having hydraulic power support for motor vehicles. A steering gear shaft having a steering wheel is connected to an input element of a steering gear. An output element of the steering gear is connected to wheels of the vehicle to be steered. Two working chambers of a servo motor of a power boosting device may be acted upon by a servo pump having hydraulic media according to an applied torque in the area of the steering gear shaft/input element. At least one sensor for detecting an angle of rotation and a torque is mounted on the steering gear shaft. A sensor for detecting a steering angle is arranged on the output element of the steering gear. This sensor may also be integrated into the angle of rotation sensors of the steering gear shaft. A return device is used for hydraulic support in returning the steering gear and the wheels to be steered after the steering wheel is released after a steering operation.
Such a power steering system is described in European published Patent Application Ser. No. 0440 638, where in addition to a complete power steering system having hydraulic power support, there is also an electric motor which is used to influence, as a function of various parameters, the reaction force detectable on the steering wheel. In addition, the electric motor is used to center the steering valve exactly at its center. Finally, a hydraulic steering axis can be returned with the electric motor by shifting the steering valve beyond its neutral position into a control position which corresponds to a reversal of the direction of steering after the steering wheel is released.
An object of the present invention is to implement the positive properties achieved with the known power steering system such as accurate valve centering and hydraulic steering axis return by simpler means without the use of an electric motor. In addition, steering accuracy is also to be improved.
The above and other beneficial objects of the present invention are providing a achieved the power steering system in which both the intensity and direction of the hydraulic power support as well as the returning of the wheels to be steered are controlled by a common electronically controlled electromagnetic valve. This electromagnetic valve may be integrated into the actual power steering system or arranged at any location outside the steering system. In addition, a torsion spring may be eliminated due to the arrangement of the steering force sensor and/or torque sensor for detecting the steering force. Extremely small control distances may be implemented with the steering force sensor and/or torque sensor, so that a high steering accuracy is achieved. The auxiliary power for the hydraulic power support is supplied exclusively by the servo pump. No electric motor is needed.
To prevent any danger in the event that an error occurs, a short-circuit valve is arranged between the steering gear shaft and the input element of the steering gear. The steering valve is otherwise located at this point in the conventional power steering systems. The short-circuit valve is designed as a rotary slide valve with a closed center, so that no connection between the two working chambers of the servo motor is possible in its neutral position. Only when an error occurs is the short-circuit valve switched to its open position so that the two working chambers of the servo motor are connected. The two working chambers are then short-circuited, so no pressure can build up in the servo motor. In this case, the steering force is obtained exclusively by mechanical operation of the steering wheel. The electromagnetic valve may be controlled easily by an electronic unit as a function of at least one parameter, such as the speed of the vehicle, the steering rate, the yaw rate, The above and other beneficial objects of the present invention are providing a. the vehicle load, the vehicle acceleration etc.
Through appropriate programing of the electronic unit and appropriate sensors, the electromagnetic valve may be controlled by signals providing information about the instantaneous position of the vehicle and desired future positions. Thus, this includes the option for automatic steering in the power steering system.
The steering force sensor and/or torque sensor may be designed as a pressure sensor or as a piezoelectric element. With such elements, the control distance may be reduced to 0 to 0.1 degree.